The field of the invention relates to methods for thin film processing, and more specifically, to modifying charge states in anti-reflection films, particularly in solar cell applications.
Thin hydrogenated silicon nitride (SiNx:H) films are typically utilized as anti-reflection coatings for the front surface of standard screen printed n+-p crystalline silicon solar cells. The films improve cell efficiency by minimizing surface recombination by passivating the dangling bonds at the interface with atomic hydrogen released during high temperature annealing and by achieving a field effect passivation due to presence of net positive charges in the film. The positive charges present in the silicon nitride film originate from the charge on a specific silicon-nitrogen dangling bond (●SiN3) known as K centers. According to charge distribution models, the positive charges originate from the formation of a thin layer of SiOxNy (<2 nm) and the charge from K centers, and are assumed to be limited within the nitride film up to roughly 20 nm away from the Si—SiNx interface. For a typical n+-p cell, the positive charges (˜5×1011 cm−2) enhance efficiency by effectively minimizing the surface recombination by way of keeping minority holes away from the surfaces of the n+ emitter.
However, the same positive charges can create a depletion or inversion region when applied to p-type doped surfaces of the cells, depending on the doping concentration. When a depletion region is created at the surface, it leads to a higher surface recombination due to presence of both type of carriers. Further, the presence of an inversion layer adjacent to metal contact regions (such as rear p-type surfaces of n+-p cell) will cause parasitic shunting thereby, degrading the cell performance. Therefore, p-doped surfaces require dielectric films carrying negative charge to shield minority electrons away from the surfaces. Currently, thin aluminum oxide (Al2O3) films with fixed negative charges are used for the rear of the p-type cells, as well as for the front of the n-type cells with p+ emitters. Although Al2O3 films provide good surface passivation, its wide industry usage is limited due to several shortcomings. First, the low rate (1-2 Å per cycle) of Al2O3 deposition using standard atomic layer deposition (ALD) methods prevents high volume manufacturing. Second, Al2O3 films have refractive indices not suitable as a standalone anti-reflection films and hence require SiNx/Al2O3 stack structures. Third, no materials are currently available that can easily penetrate Al2O3 films for achieving proper ohmic contact in subsequent solar cell processing.
Future cell architectures may rely heavily on the effectiveness of surface charges to minimize surface recombination and enhance cell efficiencies. As such, thinner substrates, lightly doped emitters and migration to n-type wafers with p-type emitters will require innovative surface passivation schemes. The above-mentioned drawbacks of current approaches make it difficult to use either as-deposited SiNx or Al2O3 films for anti-reflection coatings on all types (n-doped or p-doped) of surfaces.
Therefore, given these and other shortcomings, there is a need for a reliable and easy method to manipulate the amount and polarity of the net charge present in a dielectric, where manipulating the net charge allows application of the dielectric film to both n-doped as well as p-doped surfaces.